A conventional X-ray tube is configured so as to envelope a vacuum tube structure with an insulating oil and, vacuum at a vacuum section is maintained with a glass tube, and a cathode for emitting electrons is insulated from an anode for emitting an X-ray which irradiation of the electrons causes by the vacuum and the glass. A portion where dielectric strength is low in the configuration is an interface between the glass and the vacuum. A gas component may be adsorbed to the portion in some cases and the insulation performance considerably deteriorates if electrically conductive dust remains in the glass tube by mistake during a manufacturing process. An inside surface of the glass is mirror-finished and fully cleaned with a, solvent or the like in order to remove such contaminants, further a voltage obtained by restricting electric current with a high resistance is applied while the gas is evacuated from the glass tube, and the withstanding voltage performance is improved gradually. This process is called conditioning. By this process, the state of the withstanding voltage performance necessary for the vacuum section and the inside surface of the glass tube is obtained. The insulation of the X-ray tube is ensured by filling an exterior of the glass tube with the insulating oil in the state. However, some tubes happen to have an inferior insulation performance in rare cases and further improvement of the insulation performance is desired. In the case of a conventional X-ray tube, as shown in Patent Citation 1 (Japanese Patent Laid-open No. 2003-203591) and Patent Citation 2 (Japanese Patent Laid-open No. 2006-19223), it is attempted to improve insulation performance by homogenizing resistance at a cathode support section; forming a metal film on an inside surface of the glass tube; or roughening the inside surface of the glass tube by shot-blasting and thereby forming dents of several microns.    Patent Citation 1: Japanese Patent Laid-open No. 2003-203591    Patent Citation 2: Japanese Patent Laid-open No. 2006-19223